This invention relates to a throttle control for an aircraft turbofan engine, and more particularly to a throttle control, which eliminates dead band in the throttle control.
Typically, two different power setting parameters are used to define the speed of operation of a turbofan engine. These are fan speed at high power setting and engine core speed at idle. Historically, the transition to fan speed from core speed causes a dead band or slippage in the throttle response. The dead band is different for each engine due to the variation in engine hardware and control sensors.
The disclosure in U.S. Pat. No. 4,296,601 seeks to address this problem by controlling to a combined engine speed parameter. The combined speed parameter is comprised of core speed and fan speed. The combined parameter is correlated with power lever or throttle position so as to control fuel flow to the engine at variant power level requirements.
As indicated, many turbofan engines with electronic engine controls use fan speed as the power setting parameter for high power operation. Climb and takeoff power settings are examples of high power operation. However, at idle the same engine may use core (also known as HP shaft) speed as the power setting parameter. Typically, an equivalent idle power set fan speed is estimated for the core idle speed. The idle fan speed and the climb power setting fan provide end points for the engine throttle.
There is variability in the engine control sensors and from one engine's hardware to the next. Consequently, the estimated speed for idle may only be representative of a small number of engines. When operating at the estimated fan speed for idle some engines may have a core speed higher than the power setting core speed. To ensure all engines obtain the power setting core speed, the power set fan speed for idle is lowered below the estimated value. The lowering of the power set fan speed will ensure the engine transitions on to the core speed idle governor. However, this approach to transition from fan speed to core speed power setting often causes dead bands in the engine throttle movement.
The size of the dead band will vary depending on the individual engine characteristics. Consequently, on multi-engine aircraft the throttle for each engine will likely have a different dead band and could change as engines are replaced as part of normal maintenance.
As can be seen, there is a need for an apparatus and method that utilizes a combination of core idle speed and fan speed as parameters to ration the fuel flow to the engine to effect a smooth and continuous transition from one to the other, rather than relying on but one parameter, such as engine fan speed, to eliminate dead bands in the throttle angle.